Your first table is width="723" but the large image roses_top.jpg inside it is width="750" so the top table expands to 750 px wide.
The lower table is width="750" but does not have align="center" so the whole table with the smaller image of a rose is on the left in large screen resolutions.
If you edit the width of the top table to 750px to be the same width as the large image and add align="center" to the bottom table the bottom table will be directly under the top table.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.